Excited singlet (S1) state interactions of 6,11-dihydroxy-5,12-naphthacenequinone with aromatic hydrocarbons

Interaction in the excited singlet state (S-1) of 6,11-dihydroxy-5,12-naphthacenequinone (DHNQ) with aromatic hydrocarbons (AH) has been investigated using steady-state (SS) and time-resolved JR) fluorescence quenching measurements. In both nonpolar (cyclohexane; CH) and polar (acetonitrile; ACN) solvents, the fluorescence quenching of DHNQ by AHs is accompanied with the appearance of exciplex emissions. The emission maxima of the exciplexes correlate linearly with both ionization potentials (IP) and oxidation potentials {E(AH/AH(+))} of the quenchers (AH), indicating the charge transfer (CT) type of interaction between the S-1 state of DHNQ (acceptor) and the ground state of the AHs (donor). The kinetic details of the exciplex formation have been evaluated by analyzing the SS and TR fluorescence quenching results at different temperatures following a suitable mechanistic scheme. Picosecond laser flash photolysis (LFP) studies on the DHNQ-AH systems show a major transient absorption band in the 530 to 630 nm region along with a weak long-wavelength absorption tail. The transient lifetimes for the 530-630 nm absorption band are very similar to the exciplex lifetimes estimated from the fluorescence quenching results. At the long wavelength absorption tail, the transient lifetime could not be estimated due to very weak absorption. It is inferred that the 530-630 nm transient absorption band is mostly due to the S-1 --> S-n transition. The long wavelength absorption tail has been attributed to the anion radical of DHNQ, drawing an analogy with the anion radical absorption spectrum of 1,4-dihydroxy-9, 10-anthraquinone (quinizarin; QZ), a lower analogue of DHNQ. The picosecond LFP results largely correlate with the results obtained from the fluorescence quenching studies.